This invention relates to a method and apparatus for circulating hot air through a board drying apparatus, and more particularly to a hot air circulating method and apparatus wherein boards to be dried, mainly such boards still containing a large amount of water such as, for example, veneer, calcium silicate board, pulp cement, plaster board and fiber board are stacked on a pair of gate-like fork conveyors disposed opposite to each other, and such boards to be dried are transferred from an upper portion thereof to a lower portion thereof in separated relationship, i.e., layers of many boards, and in the process of downward voyage of the boards, a hot air is blown and circulated in the space between the pair of fork conveyors in the horizontal direction for drying the boards.
A hot air circulating method and apparatus in a board drying apparatus of this type is known as disclosed in a U.S. patent application Ser. No. 587,703 filed Mar. 8, 1984. In the method of this earlier application, hot air is merely circulated in the horizontal direction along the elongated direction of the space between a pair of fork conveyors. Although this arrangement avoids drying irregularity among boards to be dried which are transferred one after another, almost no special consideration is rendered to a single board itself to be dried. Thus, every portion of the board is contemplated to be heated homogeneously.
However, especially, in the case the board to be dried is a veneer, when hot air is applied to it, due to minute vessels in the veneer, the moisture contained in a portion of the veneer near its periphery tends to be discharged outside in a comparatively short time through the vessels and thereby dried. On the contrary, the moisture contained in the central portion thereof is not easily discharged. Accordingly, the drying speeds are largely different between the central portion and the peripheral portion. Also, the moisture content of the dried veneer is always such as to be over-dried at the right and left marginal portions with respect to the longitudinal center portion thereof. As a result, dry cracking as well as wave shaped shrinkage often occur at the right and left marginal portions, thus often deteriorating the quality of the final product.
The present invention was accomplished in view of the above mentioned problems inherent to the prior art.